1. Field of the Invention
The present invention relates to a vehicle room illuminating apparatus for illuminating a vehicle room of a passenger car, for instance.
2. Description of the Prior Art
Some examples of prior-art vehicle room illuminating apparatus will be explained with reference to FIGS. 1(A) to (C).
FIG. 1(A) shows a first example, in which a vehicle room 11 of a passenger car 4 is illuminated for passengers 5 and 6 by a room light 12 attached at roughly the center of a vehicle ceiling 13. An example of the room light 12 is disclosed in Japanese Published Unexamined (Kokai) Utility Model Appli. No. 54-3759, for instance. The room light 12 is entirely or partially covered by a light transmissible material such as resin so that the room light 12 can illuminate the vehicle room 11. In this first prior-art example, since the room light 12 is usually turned on or off with a switch, the vehicle room 11 is illuminated only when the light 12 is turned on manually at night or automatically when one of vehicle doors is open even in the day time.
In the above-mentioned prior-art illuminating apparatus, however, since the brightness of the room light 12 is kept constant irrespective of change in external brightness (i.e. illuminance or luminance), when the vehicle is running at night or in the evening along a bright shopping street or district where a number of bright roadside lamps are arranged or window illumination brightness is high long along a street, there exists a problem in that it is difficult for the passenger to well see an object such as a book even when the room light 12 is turned on. This is because the human eyes are gradually adapted to the bright external environment (called light adaptation). In other words, human eyes' visual sensitivity decreases with increasing external illuminance or luminance in a field of vision. In the above case, it has been necessary to further turn on an additional room lamp to increase brightness within the vehicle room.
On the other hand, when the vehicle is running at night long along a dark road, there exists a problem in that the passenger feels that the vehicle room is excessively illuminated and therefore dazzled. This is because the human eyes are gradually adapted to the dark external environment (called dark adaptation). In other words, human eyes' visual sensitivity increases with decreasing external illuminance or luminance in a field of vision.
FIG. 1(B) shows an example of prior-art clock display panel illuminating apparatus, in which a digital clock 14 having a display panel 15 is illuminated in such a way that the display panel brightness can be adjusted automatically according to the external brightness. That is, when the head lamps are turned off in the daytime, the brightness of this digital clock display panel is automatically adjusted higher than when the head lamps are turned on at night. This is because it is possible to allow the display panel 15 of the digital clock 14 to be well visible even in the daytime according to the adaptation of the passenger's eyes.
FIG. 1(C) shows an example of the prior-art head lamp adjusting apparatus, which comprises an external brightness sensor 16, a head lamp controller 17 for turning on or off head lamps 18 in response to signals generated from the sensor 16, and a power supply section 19 for supplying power to the head lamps 18 via the head lamp controller 17. In this third example, when the sensor 16 detects dark external environment as when the vehicle 4 comes into a tunnel, and therefore generates a lamp turn-on signal to the controller 17, the controller 17 automatically turns on the head lamps 18. On the other hand, when the sensor 16 detects bright external environment as when the vehicle 4 goes out of a tunnel and therefore generates a lamp turn-off signal to the controller 17, the controller 17 automatically turns off the head lamps 18. In this example, since the head lamps 18 are turned on or off automatically according to the external brightness, it is possible to prevent the head lamps 18 from being left turned on or off carelessly or by mistake.
In the above-mentioned second and third examples, although the illumination devices (clock display or head lamps) can be adjusted according to the external brightness, the brightness of the devices are not adjusted finely in analog fashion or multistep digital fashion under due consideration of the dark and light adaptation of the drivers' or passengers' eyes. In addition, the above-mentioned prior-art apparatus are not directly related to the vehicle room illuminating apparatus.